Salvadoran President Mauricio Funes sent a formal protest to Honduran counterpart Juan Orlando Hernandez on Wednesday to protest Honduras' construction of a heliport on a Pacific island claimed by both nations.
El Salvador has also complained to the United Nations about the "expansionist posture" of Honduras, Funes' office said in a statement.
Hernandez visited Conejo Island last Saturday to inaugurate the heliport.
Located in the Gulf of Fonseca, which is shared by Honduras, El Salvador and Nicaragua, the island is less than 1 square kilometer (0.38 sq. miles) in area.
"My government asks the Government of Honduras to abstain from the realization of these actions, asking at the same time for the immediate abandonment of the Salvadoran island of Conejo in accordance with the norms of international law," Funes said in his message to Hernandez.
The Salvadoran government has repeatedly protested the Honduran armed forces' "illegal and defacto occupation" of the island, Funes reminded his counterpart.
Conejo has been contested territory for years, though the current controversy began last September.
Honduras and El Salvador are approaching the 45th anniversary of a 100-hour-long war that claimed some 5,000 lives on both sides.
The conflict, known around the world as the "Soccer War" because it began shortly after a World Cup qualifier that El Salvador won, began July 14, 1969, and arose out of a century-old border and migration dispute.
In 1980, the two Central American neighbors signed a peace accord, but they were unable to agree on the boundaries.
The case was taken to the International Court of Justice, which set the borders Sept. 11, 1992. EFE